The present invention relates to a color image forming apparatus which transfers a color image onto a sheet of paper via an intermediate transfer belt. More particularly, it relates to the color image forming apparatus including a new mechanism which provides an intermediate transfer belt with tension, so that a number of components can be reduced and the apparatus can be downsized.
A conventional image forming apparatus employing electro-photographic technique forms an image through the following steps: First, charge a photosensitive bodyxe2x80x94an image carrying bodyxe2x80x94with a charger, second, photo-radiate the photosensitive body responsive to image information for depositing a latent image. Then develop the latent image with a developing device for forming a toner image. Finally, transfer the toner image onto a recording sheet such as a sheet of paper to form a visible image.
Recently, color images have become popular in the market, and a number of images forming apparatuses employing a tandem style have been proposed. One of these apparatuses includes a plurality of image-carrying-bodies on which a series of image forming processes are carried out. Images of respective colors including cyan, magenta, yellow and preferably black are formed on each image-carrying-body, and images of respective colors are overlaid for transferring so that a full color image can be formed. This kind of image forming apparatus includes an image forming unit for each color, and this structure is advantageous to an operation at higher speed.
FIG. 7 is a cross section illustrating an essential part of a conventional color image forming apparatus. In FIG. 7, image forming units 502, 503, 504 and 505 are disposed in this order within apparatus 501. These units 502-505 form toner images of yellow, magenta, cyan and black respectively. Apparatus 501 includes exposure devices 561, 562, 563 and 564 corresponding to units 502-505. These units 502-505 include photosensitive drums 521, 531, 541 and 551, as well as developing rollers 522, 532, 542 and 552. These photosensitive drums function as image-carrying-bodies for depositing electrostatic latent images on their surfaces by laser-beam-radiation from exposure devices 561-564. Developing rollers 522, 532, 542 and 552 attach toner supplied from toner tanks to those photosensitive drums thereby revealing the electrostatic latent images to visible toner images.
Under units 502-505 arrayed, intermediate-transfer-belt 507 circulates in an arrow marked direction passing by four first-transfer-rollers 591, 592, 593 and 594, driving roller 508, tension roller 550 and second-transfer-roller 510. The first-transfer-rollers correspond to respective photosensitive drums, and the second-transfer-roller transfers an image to a sheet of paper.
Under belt 507, paper tray 520 is disposed. Paper P supplied from tray 520 is nipped between belt 507 around second-transfer-roller 510 and urging roller 560 confronting second-transfer-roller 510. On the surface of belt 507, toner images in yellow, magenta, cyan and black are attached by drums 521, 531, 541 and 551, thereby forming a color image. This toner color image is transferred on paper P by nipping force yielded between rollers 510 and 560. Then paper P is fed to fixing device 580 thereby fixing the toner image. Finally, paper P is discharged.
Driving roller 508 and second-transfer-roller 510 are both rigidly disposed, and tension roller 550 is specifically disposed in order to provide belt 507 with tension. In FIG. 7, during a printing period, tension roller 550 is biased by a spring (not shown) and moves down right, thereby providing belt 507 with tension. During a non-printing period, on the other hand, the tension provided by roller 550 to belt 507 is released in order to avoid curling of belt 507 due to long time rolling around respective rollers.
As discussed above, a given tension is provided to belt 507, so that drums 521, 531, 541 and 551 transfer a toner image onto belt 507, and second-transfer-roller 510 transfers the toner image onto paper P.
However, tension roller 550 is prepared only for providing belt 507 with the tension, and this not only increases a number of components, but also blocks downsizing the apparatus. The cost of the apparatus tends to increase. A tension providing/releasing mechanism to/from belt 507 is additionally required, which further makes it difficult to downsize the apparatus.
Urging roller 560 is away from second-transfer-roller 510 during the non-printing period, and it moves rightward to nip paper P together with roller 510 during the printing period. During the non-printing period, urging roller 560 is kept away from belt 507 so that it will not contact with belt 507 for extending a service life of belt 507 by avoiding a local stress.
This removal mechanism of urging roller 560 from second-transfer-roller 510 is additionally required, which increases the cost and also blocks downsizing the apparatus.
The conventional color image forming apparatus not only requires tension roller 550, but also needs a driving mechanism for providing/releasing the tension to/from belt 507 as well as the removal mechanism for removing urging roller 560 from second-transfer-roller 510. These components boost the cost and blocks downsizing the apparatus.
The present invention addresses the problems discussed above and aims to provide a compact and inexpensive color image forming apparatus which utilizes movement of a transfer roller, thereby allowing to provide an intermediate-transfer-belt with tension.
The color image forming apparatus of the present invention comprises the following elements:
(a) a, plurality of image forming units which have respective photosensitive bodies carrying toner images and form images of different colored toners;
(b) a loop type intermediate-transfer-belt, disposed allowable to contact with a photosensitive body, and on which a toner image is transferred;
(c) a transfer roller, disposed in the loop of the intermediate-transfer-belt, for transferring the toner image formed on the intermediate-transfer-belt to a recording sheet;
(d) a driving roller, disposed in the loop and facing the transfer roller, for rotating the intermediate-transfer-belt.
The transfer roller provides the intermediate-transfer-belt with tension.
Another color image forming apparatus of the present invention comprises the following elements:
(a) a plurality of image forming units which have respective photosensitive bodies carrying toner images and form images of different colored toners;
(b) a loop type intermediate-transfer-belt, disposed allowable to contact with a photosensitive body, and on which a toner image is transferred;
(c) a plurality of first-transfer-rollers, disposed in the loop of the intermediate-transfer-belt, for biasing the intermediate-transfer-belt to the photosensitive body;
(d) a second-transfer-roller, disposed in the loop, for transferring a toner image on the intermediate-transfer-belt; and
(e) a driving roller, disposed in the loop and facing the second-transfer-roller, for rotating the intermediate-transfer-belt.
The intermediate-transfer-belt is rotatably held only by the second-transfer-roller and the driving roller, and only the second-transfer-roller provides the belt with tension.
These structures can provide the belt with tension by shifting the transfer-roller (the first case discussed above) or shifting the second-transfer-roller (the second case discussed above), and save a tension roller included in the conventional apparatus and reduces a number of components as well as achieves easy assembly. These structures also contribute to downsizing and reducing the cost of the apparatus.